1. Field of the Invention
This invention relates to a powdery desulfurizing agent comprising quicklime, diamide lime and calcium carbide as main ingredients. More specifically, the invention pertains to a powdery desulfurizer composition comprising quicklime, diamide lime and calcium carbide as main ingredients, which is especially effective in injection desulfurization of molten iron.
The diamide lime is a mixture consisting essentially of calcium carbonate and carbon.
The term "molten iron" as used herein, denotes a molten mass of pig iron, cast iron, steel, etc.
2. Description of the Prior Art
As is well known, desulfurization of molten iron is an important treatment for obtaining iron and steel products having excellent properties, and numerous desulfurizing agents and desulfurizing methods have been proposed heretofore.
Calcium carbide has by far the best desulfurizing ability, and desulfurizers comprising calcium carbide as a main ingredient have gained widespread acceptance. Production of calcium carbide, however, entails high electric power consumption, and it has become necessary to re-assess calcium carbide as a desulfurizer from an economical viewpoint in order to cope with the recent rise in energy cost. On the other hand, quicklime is known as one of cheaper desulfurizers. Although the iron and steel industry desires commercial utilization of quicklime, its very low desulfurizing performance has made it difficult to meet various high-level requirements in the present-day desulfurization of molten iron.
A method which comprises adding a certain powdery desulfurizing agent to molten iron and mechanically stirring the mixture and a method which comprises injecting a certain powdery desulfurizing agent into molten iron using a carrier gas are well known for desulfurization of molten iron. The injection desulfurizing method has gained widespread acceptance because of its excellent operational ease and desulfurizing efficiency. Specifically, the injection desulfurizing method comprises carrying a powdery desulfurizing agent on a stream of a carrier gas such as dry nitrogen, and injecting it into molten iron through a lance submerged in molten iron. According to a widely accepted practice of injection desulfurization, a torpedo car which has received molten pig iron from a blast furnace, for example, is stopped for a while at a desulfurizing station on its way to a steel-making factory, and a powdery desulfurizing agent is injected into molten iron in the torpedo car during this stop. Furthermore, injection desulfurization in an open ladle has been put into operation in recent years in place of the mechnically stirring desulfurizing method (e.g., the so-called KR method in an open ladle) because of its excellent operational ease and desulfurizing efficiency.
The term"injection desulfurization", as used in the present application, is a term contrastive with "desulfurization methods which involve preaddition of desulfurizers or mechanical stirring desulfurization", etc., and specifically denotes a method of desulfurization which comprises injecting a powdery desulfurizing agent together with a carrier gas into molten iron beneath its surface.
The injected desulfurizing agent escapes from the carrier gas in molten iron and makes contact with the molten iron, whereupon it reacts with sulfur in the molten iron. Then, the desulfurizing agent and/or its reaction product with sulfur rise through the molten iron and finally float as desulfurization slag on the surface of the molten iron. The molten iron is sufficiently moved and stirred by the carrier gas and/or gases which may be evolved from gas-generating substances in the powdery desulfurizing agent, and as a result, the chances of the desulfurizer to encounter sulfur in molten iron are enhanced, and the residual sulfur content in the molten iron is geometrically uniformed.
Methods for improving the desulfurizing ability of quicklime have been proposed, for example, in Japanese Laid-Open Patent Publications Nos. 38209/1979, 50414/1979, 86416/1979, and 86417/1979 which are directed mainly to size reduction of CaO crystals constituting quicklime so as to increase its contact area and thereby improve its reactivity. It has been found, however, that when quicklime treated by the methods disclosed in these prior patent documents is used in injection desulfurization of molten iron, its transportability on a stream of a carrier gas is very poor, a large amount of the carrier gas is required, and therefore injection of the quicklime in high concentrations and fine dispersion in the carrier gas is difficult, and consequently that the advantage of the finely divided CaO crystals cannot be utilized and the expected desulfurizing effect cannot be obtained. It is thus seen that although the reduction of the particle size of a desulfurizing agent has greatly to do with an increase in desulfurizing ability, its desulfurizing performance is not directly governed by its particle size, but also it is greatly affected by the transportability of the desulfurizing agent on a carrier gas.
In the injection desulfurizing method, the powdery desulfurizing agent is injected into molten iron in a form suspended in carrier gas. That part of the powdery desulfurizing agent which has escaped out of the gas bubbles of the gas stream makes direct contact with the molten iron and reacts with sulfur in the molten iron, but that portion of the desulfurizing agent which remains enclosed within the gas bubbles rises as such and floats on the surface of the molten iron without contributing to the desulfurization reaction or spurts out of the molten iron together with the gas.
In order to increase the proportion of the desulfurizer powder which participates in the desulfurization reaction and to increase its reactivity, it is desirable to minimize the amount of the carrier gas, thereby preventing the desulfurizing agent from being enclosed within the gas bubbles. The amount of the carrier gas required for injection, however, depends upon the gas transportability of the powdery desulfurizing agent, and a desulfurizing agent which has poor gas transportability requires a large amount of a carrier gas for injection. Accordingly, even a desulfurizing agent having high reactivity cannot give the desired desulfurizing effect in injection desulfurization if its transportability on a carrier gas is poor.
Furthermore, when the desulfurizing agent has poor gas transportability, great fluctuation occurs in the concentration of the desulfurizing agent in the carrier gas in injection desulfurization to cause a pulsating movement of the desulfurizer carrier gas stream which frequently becomes an operational trouble. For example, injection of an excessively large amount of the powdery desulfurizing agent into molten iron at one time, results in an excessively large amount of gas evolution at one time in the molten iron and thus increases vibration of a torpedo car, an open ladle, etc. The fluctuations in the concentration of the desulfurizing agent also can result in the desulfurizing agent blocking up the lance pipes, or the molten iron splashing vigorously out of the torpedo car, etc, and thus causing undesirable phenomena such as the pollution of the working environment, exposure to danger, and economical loss.
The present inventors made various investigations in order to improve the performance of quicklime in injection desulfurization with special attention to the poor gas transportability of various quicklime desulfurizers so far proposed, and unexpectedly found that a powdery desulfurizing composition comprising a specified amount of powdery quicklime and a specified amount of powdery diamide lime gives a complete solution to the aforesaid various problems associated with quicklime.
This powdery desulfurizer composition was applied for a patent in co-pending Japanese Patent Application No. 61261/1980 filed May 10, 1980.
It has recently been found however that the desulfurizing ability of the desulfurizer composition of the co-pending Patent Application is not sufficient in the production of ultralow sulfur iron having a sulfur content of 0.010% or less which is especially required in steel making, and a further improvement is desired.